Thromboembolic events, including stroke, are a major cause of morbidity and mortality in patients with heart failure. Even with sinus rhythm, severe heart failure confers an annual stroke risk of 4%. While historically ascribed to blood stasis, dysfunction of the endocardial endothelium is a poorly studied and potentially important contributor to intracardiac thrombus formation. Thrombomodulin (TM), a key component of the protein C anticoagulant pathway, is critical to maintaining vascular thromboresistance. We recently found that TM expression by the left atrial endocardium in rats is significantly inhibited by acute pressure overload-induced heart failure and associated with increased local thrombin generation. In addition, we found that mechanical stretch is able to potently inhibit TM gene expression in culture rat endocardial cells. Based on these data, we hypothesize that pressure-induced stretch is an important and novel regulator of endocardial TM expression. The aims of this proposal are: 1) To determine the effects of heart failure on endocardial thromboresistance. Changes in endocardial TM expression and the functional consequences on thromboresistance will be determined in the cardiac chambers of rats subjected to acute and chronic pressure overload-induced heart failure. The effects of restoring endocardial TM expression in heart failure will be evaluated using a gene transfer strategy as well as pharmacologically. 2) To define critical hemodynamic stimuli and signaling pathways regulating TM expression. We will use endocardial cells placed into a novel ex vivo perfusion system to differentiate the effects static versus pulsatile stretch, to determine if shear modulates the effects of stretch and to define critical stretch-activated signaling pathways that mediate TM inhibition. 3) To determine the transcriptional mechanisms by which stretch regulates TM gene expression. We will use promoter-reporter constructs to localize stretch-responsive TM promoter elements and use chromatin immunoprecipitation and other molecular techniques to determine the effects of stretch on the expression and activity of transcription factors that regulate constitutive TM expression. Realization of the above specific aims will provide valuable insights into the pathobiology underlying thromboembolic complications associated with heart failure and may lead to novel therapeutic options for their prevention.